An exhaust gas treatment device of a diesel engine in a conventional practice is configured to have an exhaust gas separator disposed on an exhaust gas flow passage to allow PM in exhaust gas to be localizedly positioned and thus to separate the exhaust gas into EGR gas containing the localizedly positioned PM and emission gas as the rest of the exhaust gas, thereby recirculating the EGR gas to a combustion chamber and emitting the emission gas to the air (For example, see cited prior art 1).
According to the conventional exhaust gas treatment device of the diesel engine, the PM contained in the EGR gas is burnt and treated by the combustion heat of the combustion chamber during the operation of the engine, and therefore, there is no need for the installation of the DPF. Alternatively, through the use of the DPF with the exhaust gas separator, the DPF can be small-sized. Since there is no need to reserve a large amount of PM in the exhaust gas separator, the size of the exhaust gas separator can be smaller than that of the DPF. In case where the exhaust gas separator is used in place of the DPF and in case where the DPF is used with the exhaust gas separator, advantageously, a small-sized diesel engine can be made.
According to the conventional practice, an electrode is disposed at the center of the upstream side of the exhaust gas separator, and an electrode is disposed on the outer peripheral wall of the exhaust gas separator, thereby charging the PM in the exhaust gas by means of corona discharge occurring between the electrodes. Also, an inner cylinder is disposed at the center of the downstream side of the exhaust gas separator, and the EGR gas is collected outside the inner cylinder, while the emission gas is being collected inside the inner cylinder. Since an emission gas entry inlet formed on the inner cylinder is opened along the upstream end of the inner cylinder, some problems have occurred.